It is known in the art relating to engine valve gear to provide various means for varying valve timing as desired for the control of engine performance and efficiency. Among the various types of variable valve timing devices employed have been camshaft phasing devices, often in the form of drive pulleys and the like incorporating phase changing means for varying the phase between a rotatably driving input member such as a gear, pulley or sprocket and a rotatably driven output member such as a camshaft. Among the pertinent prior art are mechanisms having splined pistons which are hydraulically actuated against a spring to vary the phasing of outwardly and inwardly engaged drive and driven members. Such arrangements are shown for example in U.S. Pat. Nos. 4,231,330 Garcea and 4,811,698 Akasaka et al.
Garcea FIG. 1 and Akasaka et al FIG. 3 also disclose prior hydraulic systems for actuating phase adjusting devices. In these systems, the hydraulic actuator is continuously connected with the engine oil pressure system and a drain valve is used to control pressurization of the actuating chamber. When the drain valve is open, oil delivered to the actuator is drained to the sump or other oil source so the chamber is not pressurized and the device is not actuated until the drain valve is closed. Such systems are simple but have the disadvantage of requiring a continuous oil flow whenever the drain valve is open which decreases operating efficiency and may reduce engine oil system operating pressures or require a larger oil pump capacity than would otherwise be needed.